Sigeo Morimoto and Masato Koyama "Application of Adaptive Model Following Control to Motor Speed Control System" (a paper of Semiconductor Power Converter Committee for Japan Electrical Engineer's Society, SPC-86-2 (1985) pp 11-20) and Tatsuro Hasegawa, Ryoichi Kurosawa, Hiromi Hosoda and Kikuo Abe "A Microcomputor-Based Motor Drive System with Simulator Following Control" (IEEE, IECON '86, pp 41-46) disclose speed control apparatuses for a motor suitable for driving an oscillatory load which comprises a motor model consisting of a motor torque model and a motor speed model connected in cascade, a current instruction signal is applied both a current control circuit and the motor model, wherein an output from the motor speed model and an actual motor speed are compared and a compensating signal proportional to the difference therebetween is added to the current instruction signal, which is equivalent to a torque instruction signal so that a corrected current instruction signal is formed for the motor current control circuit to achieve an adaptive model following control.
The above-described prior art is effective, in that the addition of the compensating signal, which is proportional to the difference in speed between the model and the actual motor, to the current instruction signal for the motor speed control apparatus enables a stable speed control by suppressing oscillation even if the motor drives an oscillatory load. However, when a response of a current control system in the motor speed control apparatus is slow, which is equivalent to a torque control system, and to which a current instruction signal is applied, the delay in response disadvantageously leads to reduction in the oscillation suppressing effect.